Edge case: links in container node + tests

src/main/java/it/inaf/oats/vospace/FileServiceClient.java

src/main/java/it/inaf/oats/vospace/LinkService.java

...test/java/it/inaf/oats/vospace/FileServiceClientTest.java

src/test/java/it/inaf/oats/vospace/LinkServiceTest.java

src/test/resources/test-data.sql